1. Field of the Invention
This invention relates to an electrophotographic lithographic printing plate precursor made by an electrophotographic system and more particularly, it is concerned with an improvement in a photoconductive layer forming composition for the lithographic printing plate precursor.
2. Description of the Prior Art
A number of offset masters for directly producing printing plates have hitherto been proposed and some of them have already been put into practical use. Widely employed among them is a system in which a photoreceptor comprising a conductive support having provided thereon a photoconductive layer mainly comprising photoconductive particles, for example, of zinc oxide and a binder resin is subjected to an ordinary electrophotographic processing to form a highly lithographic toner image on the surface of the photoreceptor, followed by treating the surface with an oil desensitizing solution referred to as an etching solution to selectively render non-image areas hydrophilic and thus obtain an offset printing plate.
Requirements of offset masters for obtaining satisfactory prints include: (1) an original should be reproduced faithfully on the photoreceptor; (2) the surface of the photoreceptor has affinity with an oil-desensitizing solution so as to render non-image areas sufficiently hydrophilic, but, at the same time, has resistance to solubilization; and (3) a photoconductive layer having an image formed thereon is not released during printing and is well receptive to dampening water so that the non-image areas retain the hydrophilic properties to be free from stains even upon printing a large number of prints.
It is known that these properties are affected by the ratio of zinc oxide to a resin binder in the photoconductive layer, For example, if the ratio of a binder resin to zinc oxide particles is decreased, oil-desensitivity of the surface of the photoconductive layer is increased to reduce background stains, but, on the other hand, the internal cohesion of the photoconductive layer per se is weakened, resulting in reduction of printing durability due to insufficient mechanical strength. If the ratio of a binder resin to zinc oxide particles is increased, on the other hand, printing durability is improved, but background staining becomes conspicuous. It is a matter of course that the background staining is a phenomenon associated with the degree of oil-desensitization achieved and it has been made apparent that the oil-desensitization of the photoconductive layer surface depends on not only the binder resin/zinc oxide ratio in the photoconductive layer, but also the kind of the binder resin used to a great extent.
For particular use as an offset master, occurrence of background stains due to insufficient oil-desensitivity presents a serious problem. In order to solve this problem, various resins for binding zinc oxide have been proposed, including resins having a weight average molecular weight Mw 1.8-10.times.10.sup.-4 and a glass transition point Tg 10.degree.-80.degree. C., obtained by copolymerizing (meth)acrylate monomers and other monomers in the presence of fumaric acid in combination with copolymers of (meth)acrylate monomers and other monomers than fumaric acid, as described in Japanese Patent Publication No. 31011/1975; terpolymers each containing a (meth)acrylic acid ester unit having a substituent having carboxylic acid group at least 7 atoms distant from the ester linkage, as disclosed in Japanese Patent Laid-Open Publication No. 54027/1978; tetra- or pentamers each containing an acrylic acid unit and hydroxyethyl unit, as disclosed in Japanese Patent Laid-Open Publication Nos. 20735/1979 and 202544/1982; terpolymers each containing a (meth)acrylic acid ester unit having an alkyl group having 6 to 12 carbon atoms as a substituent and a vinyl monomer containing carboxylic acid group, as disclosed in Japanese Patent Laid-Open Publication No. 68046/1983; and the like. These resins function to improve the oil-desensitivity of photoconductive layers. Nevertheless, evaluation of such resins as noted above for improving the oil-desensitization indicates that none of them is completely satisfactory in in terms of stain resistance, printing durability and the like.
Furthermore, Japanese Patent Laid-Open Publication Nos. 232356/1989 and 261657/1989 describe that addition of resin grains containing hydrophilic groups to the photoconductive layer is effective for improving the water retention.
It has been confirmed that the water retention is largely increased by improving these photoconductive compositions. However, detailed estimation thereof as a lithographic printing plate precursor tells that in some cases, the electrophotographic properties, in particular, dark charge retention, photosensitivity, etc. are changed or deteriorated when the ambient conditions are changed in a high temperature and high humidity or in a low temperature and low humidity, and a stable and good reproduced image cannot thus be obtained. Consequently, the use of these photoconductive compositions for a printing plate precursor results in deterioration of a print image and decrease of the effect of preventing background stains.
When using the scanning exposing system using a semiconductor laser beam for an electrophotographic lithographic printing plate precursor as a digital direct lithographic printing plate precursor, furthermore, higher performances are required for static properties, in particular, dark charge retention and photosensitivity, since the exposing time is longer and the exposing intensity is more restricted than in the overall and simultaneously exposing system of the prior art using visible rays.
On the contrary, in the above described precursor of the prior art, the electrophotographic properties are deteriorated and real copy images tend to meet with occurrence of background stains and disappearance of fine lines or batterig of letters, so that when printing is carried out using it as a lithographic printing plate precursor, the image quality of a print is lowered and there is not found the effect of preventing background stains by improvement of the hydrophilic property of non-image areas of a binder resin. The present invention aims at solving the above described problems of the electrophotographic lithographic printing plate precursor of the prior art.